1. Field of the Invention
The present invention relates to an optical information recording method, in which information is recorded and played back with a laser, and to an optical information recording medium used in the method. In particular, the invention relates to a heat-mode optical information recording medium on which information is recorded by irradiating the medium with a shortwave laser having a wavelength of 450 nm or shorter, and to an optical information recording method using the medium.
2. Description of the Related Art
A CD-R is a writable, optical information recording medium (optical disc) on which information can be written only once by irradiation with a laser, and has conventionally been known. A CD-R typically comprises a transparent disc substrate having successively disposed thereon a recording layer including an organic dye, a reflective layer including metal such as gold, and a protective layer made of resin. Information is recorded on the CD-R by irradiating the disc with a near infrared laser (i.e., a laser generally having a wavelength of around 780 nm). Specifically, the portion of the dye recording layer that is irradiated absorbs light, whereby temperature increases at the irradiated portion. The increase in temperature produces a physical or chemical change (e.g., formation of pits) to thereby alter the optical characteristics of the irradiated portion, whereby information is recorded. The information thus recorded on the optical disc is ordinarily read (played back) by irradiating the optical disc with a laser having the same wavelength as that of the laser used to record the information and detecting a difference in reflectance between the region of the dye recording layer whose optical characteristics have been changed (recorded portion) and the region of the dye recording layer whose optical characteristics have not been changed (unrecorded portion).
In recent years, there has been a demand for optical information recording media having higher recording density. In response to this demand, digital versatile disc-recordable discs (DVD-Rs) have been proposed (e.g., see the supplemental issue of Nikkei New Media entitled “DVD”, 1995). DVD-Rs include a transparent disc substrate disposed with a guide groove (pre-groove) for tracking irradiated laser light and having a width that is equal to or less than half (0.74 to 0.8 μm) the width of the pre-groove of a CD-R. DVD-Rs typically comprise two such transparent disc substrates that each have successively disposed thereon a dye recording layer, a reflective layer, and, if needed, a resin protective layer, with the discs being adhered together so that the recording layers face inward. DVD-Rs may also comprise one such transparent disc substrate (disposed with the aforementioned layers) adhered together with a protective substrate having the same disc shape as the transparent disc substrate, with the recording layer similarly facing inward. Information is recorded onto or played back from the DVD-R by irradiating the DVD-R with a visible laser (ordinarily having a wavelength of 630 nm to 680 nm), and it is possible to record at a higher density than a CD-R.
Recently, high-vision television and networks such as the Internet have rapidly become more widespread. In addition, with the start of HDTV (High Definition Television) broadcasting being near at hand, the demand for large-capacity optical recording media capable of recording visual information easily and inexpensively is rising. While DVD-Rs have to a certain extent secured a position as large-capacity recording media, they still do not possess a sufficiently large recording capacity to meet the demands of the future. For this reason, development of optical discs in which recording density is improved by using shortwave lasers and that have even greater storage capacity continues to advance. For example, Japanese Patent Application Laid-Open (JP-A) Nos. 4-74690, 7-304256, 7-304257, 8-127174, 11-53758, 11-334204, 11-334205, 11-334206, 11-334207, 2000-43423, 2000-108513, 2000-113504, 2000-149320, 2000-158818, and 2000-228028 disclose methods for recording information on and playing back information from optical recording media including an organic dye recording layer by irradiating the recording layer with a laser having a wavelength of 530 nm or less. These methods specifically propose irradiating, with a blue (wavelength of 430 nm or 488 nm) or blue-green laser (wavelength of 515 nm) laser, optical discs including a recording layer comprising a dye such as a porphyrin compound, an azo dye, a metallic azo dye, a quinophthalone dye, a trimethynecyanine dye, a dicyanovinylphenyl skeleton dye, a coumalin compound, and a naphthalocyanine compound.
Also, in view of compatibility with existing CD-R systems, optical information recording media with which it is possible to record and play back information using lasers in mutually different wavelength regions have been proposed. For example, JP-A Nos. 2000-141900, 2000-158816, 2000-185471, 2000-289342, and 2000-309165 propose recording media with which it is possible to record and play back information with any laser by mixing dyes used in CD-Rs and dyes used in DVD-Rs together.
However, the present inventors have ascertained that further improvement is needed because, when information is recorded on the above-described conventional optical discs by irradiating the discs with a shortwave laser having a wavelength of 450 nm or less, recording characteristics, such as reflectance and percentage modulation, are not satisfactory and storage stability is insufficient. In particular, recording characteristics of the above-described optical discs drop when the discs are irradiated with a laser having a wavelength in the vicinity of 405 nm.